For the better part of the past twenty-five-hundred years, the political imagination of the Western world has been enchanted by an image of communal unity. The image emerged from the experience of life in the city-states of the ancient world, where philosophers suggested that a political community is a collective enterprise animated by a comprehensive vision of the highest human good—and that the content of this vision is determined by the individuals, families, factions, or classes that rule the community. Over the centuries, the image grew more elaborate. At some points, the community was described as a “ship of state” commanded by a brave and virtuous captain; at other times, it was a “body politic” whose head wisely guides the motions of its limbs. But in every case, the image conveyed the view that politics is a contest over who will win the honor of standing at the center of a political community, serving as the part that fundamentally shapes its character as a whole.

The first liberals, writing in the bloody aftermath of the Protestant Reformation, broke from this assumption, treating disagreement and discord about the highest good as a given and then proposed that civil peace in a deeply divided society could best be established and maintained by excluding as much as possible the most divisive questions—metaphysical questions—from political life. Citizens would still have strongly held views about the highest good, but they would no longer presume that their neighbors or the political community as a whole would collectively endorse those views. Instead of articulating the community’s shared vision of metaphysical truths, politics would focus on the more mundane task of securing the (economic and social) preconditions of individuals and groups pursuing the highest good, however they happened to define it.

Perhaps the most famous example of this liberal orientation can be found in the Declaration of Independence and its ringing invocation of a natural right of individuals to pursue happiness. The document’s silence about the content of happiness and about what actions or ways-of-life are conducive to happiness would have been unthinkable in earlier forms of political thinking. But for a political liberal like Thomas Jefferson, the silence was an unavoidable outgrowth of the lack of a sufficient consensus regarding humanity’s highest ends. Where such a consensus is lacking, it is foolish to expect more from politics—to expect the state to articulate and enforce a single, comprehensive notion of the highest good. Far from restoring the sense of spontaneous unity and shared meaning we like to see in pre-liberal forms of political life, such efforts would inevitably end up using state power to impose the values and beliefs of one part of a deeply divided community on its other parts.

But liberalism doesn’t just provide tools that make it possible for a society to manage pluralism. Once individuals and groups are granted the freedom to pursue the good as they wish, unburdened by the threat of political coercion, the political community becomes even more pluralistic than it already was—and in different ways than it already was. Visions of the highest good proliferate, setting individuals and groups off in different directions, pursuing happiness along divergent paths. In such a society, pluralism even seeps into individual lives, creating new, multifaceted forms of personal identity as well as a wide array of interwoven, rule-based social relationships. Six days a week my mailman delivers mail to my home—not because of his benevolence or public spiritedness, but because he abides by the rules of his job. But at the same time, those rules do not fundamentally define him; the task of delivering the mail is merely a part of his life—merely one aspect of his identity. He may also be a devoted husband and father of two young children, a lapsed Catholic, an avid football fan, a member of the town council, an occasional bowler, a staunch Republican, an ex-Deadhead, a regular and enthusiastic viewer of Top Chef, a proud third-generation Italian American, and so forth, through all of the disparate goods he pursues in the multiple aspects of his private and public lives. Each of these aspects and each of these pursuits tell us something about his identity, but none of them should be confused with its essence, with its core, with who he is as a whole, which is somehow the totality of these various aspects and pursuits, and their complicated interrelations—just as a liberal society is somehow the totality of the various pursuits of every individual within it.

These are some of the overlapping centrifugal tendencies of modern liberalism—of a society devoted to individual freedom, of a political community in which questions concerning the highest good have been displaced from a position of public primacy. It is, in a word, a centerless society—and one that grows ever more centerless, more differentiated, more pluralistic over time. Not that all liberal societies are equally centerless. In Europe, where the liberal idea was born, cultural and ethnic homogeneity have acted as a brake on the process of social differentiation, as they have to an even greater extent in Japan and other Asian nations. It is in the United States, with its large population, vast size, highly dynamic capitalist economy, and ethnically heterogeneous population united by little besides the liberal creed classically expressed in the Declaration of Independence, that centerlessness has been taken to its greatest extreme, creating a society with no single center and no part that can claim unchallenged supremacy over the country as a whole.

Nowhere is American centerlessness more apparent than in religion. In the nearly four centuries since the first dissenting Protestants arrived in New England seeking the freedom to worship, religion has proliferated far beyond anyone’s conceivable prediction. Thanks in large part to the liberal bargain at the heart of the First Amendment—which requires religious groups to give up their desire to seek the establishment of a particular religion in return for the virtually unlimited free exercise of faith—the Protestant churches brought to American shores from the Old World have splintered countless times, giving birth to multitudes of new sects, and even entirely new religions. But the story of religion in the United States is hardly limited to Protestantism, or even to Christianity. Roman Catholicism has taken root and flourished, becoming in the course of the twentieth century the single largest Christian denomination in the country, while Judaism has thrived to such an extent that the Jewish experience in America has raised questions for the Zionist project, showing at long last that it is possible for Jews to live and prosper in peace and safety in a majority-Christian nation. And then there is Unitarianism, Eastern Orthodoxy, Islam, Buddhism, Hinduism, sundry New Age faiths, and thousands of smaller groups, each of them worshipping, praying, and preaching in their own ways, with each of their members free to dissent and set out on a new spiritual path, which they often do.

It is an enormous human achievement—allowing people to seek and find religious meaning in their lives without fear of government coercion and in the process creating a centerless society of mindboggling diversity and complexity. And yet religion is also where the centerless society encounters its greatest, and gravest, test.

The trouble comes from specifically traditionalist religion and its belief in comprehensive spiritual and moral truths—truths that sometimes clash with the preconditions of liberal politics and social centerlessness. When the urge to seek purity through social and political withdrawal becomes more than a personal quest and starts to be treated as a preliminary step toward cleansing the nation as a whole of spiritual contaminants, it raises the specter of theologically inspired conflict and oppression. When authoritarian elements of religion seep into the political world, believers find themselves torn between theological and political loyalties. When the faithful denounce the pursuit of knowledge about the world, they produce a population incapable of acting as thoughtful and informed citizens. When religious groups view the nation’s politics and history through the lens of divine providence, they promise a false clarity that simplifies and distorts our understanding of the country’s actions in the world. When traditionalists attempt to use the law to impose their vision of sexual morality on the nation as a whole, they show that they have failed to comprehend the ineradicably pluralistic character of a centerless society. Even the rejection of God, when it becomes a counter-religion with its own absolute moral imperatives, can test the liberal order by fostering social intolerance, promoting the idea of coercive secularization, and inflaming fears among already-paranoid elements of the religious right.

Non-liberal governments frequently outlaw groups that fundamentally challenge them. But liberalism is committed to upholding religious freedom. That commitment has given rise to an alternative view, elegantly captured in the famous metaphor of a wall of separation between church and state. In the separationist ideal, religion is completely cordoned off from politics but left free to thrive in the private sphere of people’s lives. It is a noble vision—one that powerfully expresses the core liberal conviction that religion and politics both benefit from keeping out of each other’s business. But the metaphor is also deceptive, implying as it does that it’s possible to construct a settled, fixed, permanent, impenetrable legal barrier between the parties.

A better image might be an endlessly shifting skirmish line, with each party constantly pushing back against the incursions of the other, adjusting its arguments and tactics as circumstances and situations change over time. On one side, each religious group attempts to take and hold territory it views as vital to its survival and its theologically inspired vision of the world. (Sometimes the religious groups even band together to fight for a common cause, as traditionalist Protestants, Catholics, Mormons, and other churches have repeatedly done over the mainly sexual issues at the heart of the post-1960s culture war.) Meanwhile, on the other side of the line the liberal state seeks to balance the freedom of innumerable competing individuals and groups against each other as well as against the common good of the centerless society as a whole. Unlike a wall, which once built can only be either defended or torn down, a skirmish line is dynamic, changing its shape, size, and location as a direct result of variations in hostility between the competing parties and in direct response to the relative strength of the argumentative firepower deployed by each side. And that, it appears, is our fate—and the liberal promise: to fight out our theological-political differences without end, one battle at a time, using reasons instead of rifles.

The conflict between politics and religion will not end anytime soon, or ever. But the intensity of the clash might be diminished by reflection on the more-or-less permanent tensions between devout belief and the liberal order.

History shows us that traditionalist religion and various forms of non-liberal government (theocracy, absolute monarchy) can be compatible. The same can be said of strident atheism and totalitarianism. Conversely, when religion is liberal—when it makes few supernatural claims, when it is doctrinally minimal, and when it serves primarily as a repository of moral wisdom—it can play a significant public role in a liberal society. This is what America’s constitutional framers—most of them liberal Christians or deists who believed that God plays little-to-no providential role in the world—meant when they spoke about religion contributing to the stability and flourishing of the nation’s liberal system of government. In our own time, sociologist Christian Smith has argued that many American teenagers and young adults espouse similarly lukewarm religious views, which he dubs Moralistic Therapeutic Deism. Whether such beliefs are politically beneficial or merely innocuous, they pose no serious challenge to the liberal order of the United States.

The relationship between traditionalist religion and liberal politics is far more contentious. In a liberal political community, devout believers will be free within very broad limits to live their faith, and in most cases they will be quite capable of fulfilling the (relatively limited) ordinary duties of citizenship: voting, paying taxes, occasionally sitting on juries, serving in the armed forces when called to defend the nation in times of war. Yet even in these seemingly easy cases, there will sometimes be difficulties for the most intensely religious. The Amish in the United States refuse to serve in the military, for example, as do Haredi Jews in Israel. Likewise, the ritual observances of many faiths may at times make it difficult for their members to vote or fulfill jury duty.

Tensions increase exponentially as we approach the most intense forms of piety and the most exalted forms of citizenship (which involve serving in high political office). A deeply devout Christian—someone who places his faith at the center of his life—will tend to think of himself first and foremost as a member of the One True Church working toward the establishment of the Kingdom of God under Jesus Christ, if not in this life, then in the next. His ultimate loyalty will be to Christ, just as the ultimate loyalty of the most observant Jew will be to God and the Torah, while a Muslim’s will be to Allah and the Quran. Liberal citizenship at its peak, by contrast, requires devotion to the liberal institutions and democratically enacted laws of the political community above all else. That’s why American presidents and other high officials swear an oath to uphold the Constitution of the United States and not natural or divine law of any kind.

These divergent loyalties might not come into direct conflict every day, but they nonetheless stand in deep and abiding tension with one another, forever threatening to pit the theological duties of the devout believer against the political duties of the citizen. And the tension increases the closer we come to the peak of political power. A devout Mormon can serve as the mayor of Provo, Utah, without failing the religious test, for example, both because the population of Provo is 88 percent Mormon and because the political power wielded by mayors is checked by numerous higher-ranking officials, including the state governor and legislature as well as several state and federal courts. The same dynamic applies, in reverse, to a doctrinally committed atheist running for the mayor of San Francisco, Seattle, or other similarly secular urban area. A devout Mormon or proselytizing atheist running for president of the United States, however, is very different and far more troubling prospect.

It is possible for someone of liberal or moderate belief to be a great president—because his faith will make few potentially uncompromising, illiberal demands on him. But the same cannot be said of the most devout believers, who face a stark choice. Either they can practice the art of drawing distinctions between their piety and the nation’s politics—or they can refrain from seeking high political office. What will be never be possible is a theological-political synthesis. As long as the United States remains a liberal nation with a centerless society, traditionalist religion at its peak will fail to harmonize with politics at its peak. Our saints will not be statesmen and our statesmen will not be saints. That is perhaps the most important and enduring lesson to be learned from the religious test.

Damon Linker is a senior writing fellow at the Center for Critical Writing at the University of Pennsylvania and the author of The Theocons and The Religious Test.